A critical step in the rejection process is the migration of anti-donor effector or memory T cells to the transplanted organ. Interrupting this step should prevent or weaken rejection, but safe clinical strategies to do so are not available. During the previous funding cycle we identified a key pathway by which anti-donor effector and memory T cells migrate to vascularized organ allografts. We demonstrated that donor antigen, presented by either graft endothelial cells or bone marrow-derived antigen presenting cells, is necessary and sufficient for the firm adhesion and trans-endothelial migration of anti-donor T cells. Signaling via Gi-coupled chemokine receptors was not required. In this grant renewal, we propose to investigate two novel aspects of antigen-driven T cell migration: (1) the role of graft antigen presenting cells, and (2) the role of TCR-triggered signaling pathways that cause T cell adhesion and transmigration. To accomplish these aims we will utilize heart and kidney mouse transplantation models, gene knockout mice in which antigen presentation or key signaling pathways are disrupted, and intravital imaging techniques that track migrating T cells in real time. The proposed studies are innovative and significant because they represent a shift from the classical, chemokine-dependent migration paradigm and provide novel opportunities for interrupting activated T cell migration in a specific and safe manner.